Devices that convert movement of a user into a voltage may be used for energy harvesting. Energy harvesting is a process by which energy is derived from external sources and converted to electrical energy. An example of an external source is the kinetic energy of a moving person. Piezoelectric materials have the ability to transform mechanical strain energy into electrical charge distribution and are used to convert motion from the human body into electrical energy. For example “Energy Scavenging with Shoe-Mounted Piezoelectrics”, by Nathan S. Shenck and Joseph A. Paradiso, MIT Media Laboratory, Responsive Environments Group, http://www.rst2.edu/njheps/resources/energy scavenging.pdf, discloses energy scavenging with shoe mounted piezoelectric materials.